1. Field of the Invention
The present invention relates to a device for transferring a toner image from a photosensitive means onto a recording paper, and then separating the recording paper from the photosensitive means.
2. Description of the Prior Art
In an image-forming apparatus such as electrophotographic copying machines and laser printers, an electrostatic latent image is formed on a photosensitive drum, and the latent image is developed with toner to form a toner image which is then transferred to a recording paper by a transfer charger. The recording paper bearing the toner image is separated from the photosensitive drum.
In order to explain the background, reference will be made to FIGS. 15 and 16:
The known transfer charger is disposed under a photosensitive drum 30 which is rotated in the direction of arrow A. By the time when a particular area on the photosensitive drum 30 reaches a point in opposition to the transfer charger 81, a toner image is formed thereon. The recording paper 50 is fed in the direction of arrow B between the photosensitive drum 30 and the transfer charger 81. The transfer charger 81 includes a shield casing 81a in which a charging wire 81b is provided in tension to which an electric power is supplied so as to discharge. Thus, the recording paper 50 is charged with a reverse polarity to that of the toner image. In this way the toner image on the photosensitive drum 30 is transferred onto the recording paper 50.
The recording paper 50 bearing the toner image transferred from the photosensitive drum 30 sticks to the photosensitive drum 30 and rotates together. Therefore, it is necessary to electrically separate the recording paper 50 from the photosensitive drum 30. To this end, a discharger brush unit is disposed downstream of the transfer charger 81 in the direction in which the recording paper 50 is fed.
FIG. 16 shows the discharger brush unit as an example of dischargers; the brush unit includes a number of electroconductive bristles 82 supported by an insulating support 83 in such a manner as to be spaced from one another. The tip portions of the bristles 82 are kept out of contact with the recording paper 50, and bias voltage is applied to the bristles 82; for example by connecting the bristles 82 to the ground.
The bristles 82 allow the charging wire 81b of the transfer charger 81 to discharge through the recording paper 50, thereby removing the charge of the recording paper 50. A feed roller 84 and a feed belt 85 are disposed downstream of the discharger brush unit in the direction in which the recording paper 50 is fed.
Other kinds of discharger can be used; for example, an electroconductive member which has a saw-toothed edge and is placed near the recording paper 50 with the saw-toothed edge toward the recording paper 50. When an electric power is supplied to the charging wire 81b of the transfer charger 81, an electric current flows through the photosensitive drum 30, the shield casing 81a of the transfer charger 81, and the discharger such as the bristles 82.
The charging wire 81b of the transfer charger 81 is supplied with an electric power through a constant-voltage transformer or a constant-current transformer so as to secure a constant voltage or a constant current.
However, when a constant voltage is applied to the charging wire 81b of the transfer charger 81 through a constant-voltage transformer, the voltage applied to the charging wire 81b remains consistent in spite of variations in impendance due to ambient changes. This results in the reduction of the electric charge required for transferring the toner image from the photosensitive drum 30 onto the recording paper 50. As a result, the toner image is not properly transferred from the photosensitive drum 30 onto the recording paper 50.
When the constant current is supplied to the charging wire 81b, the electric charge required not only for transferring the toner image but also for removing the electric charge from the bristles 82 is controlled only for maintaining a constant current. Any variations in impendance in the recording paper discharger are likely to unfavorably affect the transfer of a toner image and/or the removal of charge of the recording paper.
The discharging bristles 82 and the saw-toothed discharger are liable to deformation or damage by rough contact with a repairing tool or a cleaning tool when the copying machine is repaired or cleaned. If any deformation or damage occurs, their discharging performance is decreased. The individual bristles 82 are likely to separate from the support 83 and the separated bristles come into contact with the charging wire 81b, thereby causing leakage therethrough.